1. Technical Field
The present invention relates generally to sealing perforations in a wellbore. More specifically, the disclosure relates to multi-layer ball sealers having a deformable layer to allow the ball sealers to better adapt to different perforation shapes thereby providing better sealing.
2. Background of the Invention
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
It is a common practice in the petroleum industry to complete wells that have been drilled into the subsurface of the earth by placing into the well a cylindrical casing and cementing the casing into the well. The casing, and surrounding cement, provides fluid isolation between the well and the formation surrounding the well. To introduce fluid flow between the interior of the casing and the surrounding formation at desired locations in the well, the casing is perforated.
It may become desirable or necessary during the productive life of a reservoir to improve the fluid flow from the reservoir into the well through techniques collectively known as reservoir stimulation. Two commonly used techniques are hydraulic fracturing and chemical stimulation.
Hydraulic fracturing is a process whereby a subterranean hydrocarbon reservoir is stimulated to induce a highly conductive path to a formation, increasing the flow of hydrocarbons from the reservoir. A fracturing fluid is pumped at high pressure to crack the formation, creating larger passageways for hydrocarbon flow. The fracturing fluid may include a proppant, such as sand or other solids that fill the cracks in the formation, so that the fracture remains open when the fracturing treatment has been completed and the high pressure is released.
Chemical stimulation is a process wherein flow through passageways in the formation is improved by dissolving materials in the formation, for example, by pumping acid through perforations in the casing into the formation.
In a trivial case, such as in a well in which only one zone has been perforated or in which treatment can be applied through all perforations, no zonal isolation is necessary. However, in wells with many perforations or multiple pay zones, it is often crucial to a successful reservoir stimulation operation to accurately and effectively isolate one zone for which treatment is to be applied from other zones where treatment is not to be applied. One reason for the need of effective zonal isolation is that treatment fluids, if applied equally to all perforations, are more likely to flow into zones with high permeability rather than into zones with poor permeability, i.e., the zones where permeability-improvement is desired. Therefore, it is desirable in such circumstances to divert the treatment away from the high-permeability zones, so the treatment, whether hydraulic fluid or chemical, does not flow to these zones instead of to the zones for which the treatment is desired.
Zonal isolation is achieved by employing a diversion technique. One approach involves the use of perforation ball sealers. Ball sealers are, as the name suggests, spherical shaped objects which are meant to seal the perforations and prevent or inhibit fluid from within the wellbore from leaking through the perforations into the formation.
Ball sealers are typically introduced into the well at the surface and are carried down the well with the treatment fluid. A positive pressure differential is maintained between the well and the formation surrounding the well. When a ball sealer encounters an open perforation with such a pressure differential, i.e., higher pressure in the well than in the formation, the ball sealer seats itself on the perforation and is held in place by the positive pressure differential.
It is desirable that the ball sealers produce an effective seal without being permanently lodged in the perforation or the formation. Therefore, ball sealers are advantageously sized so as to maximize their sealing potential without entering into the perforation.
Ball sealers exist in a variety of diameters and densities to be applicable for different environments and to be size-appropriate for the entry holes the ball sealers are intended to seal. Ball sealers are either soluble or non-soluble.
Perforations are often shot using gun arrays that are positioned off center in the casing. A commonly used perforating gun with 90 degree shot facing produces at least two perforations with oval-shaped openings. Such ovality inherently results in a poor seal between a spherical ball sealer and the perforation. Even though perforation quality has improved in recent years, there are still perforations that have sufficiently burred openings that spherical ball sealers provide poor seals.
The perforation openings may also deteriorate before the ball sealers seat on the perforation opening. Because fluid flow tends to follow the path of least resistance, significant fluid flow may be expected through perforations that are to be sealed before ball sealers seat. Treatment fluids are often very abrasive. Therefore, this fluid flow may cause erosion of the perforation before the ball sealers seat on the opening.
Poor sealing presents problems. For one, treatment fluids are often very abrasive. If there is a fluid flow past a seated ball sealer there will likely be a very quick erosion of the ball sealer further limiting its capacity for sealing the perforation and thus eliminating the desired diversion.
Early ball sealers were usually constructed as spherical shapes with solid or hollow cores covered by a soft, thin coating applied to the surface. See for example, U.S. Pat. No. 4,102,401, to Erbstoesser, entitled Well Treatment Fluid Diversion with Low Density Ball Sealers, issued Jul. 25, 1978. Erbstoesser describes a ball sealer having an inner core of a syntactic foam (or alternatively, a thermoplastic such as polymethylpentene) covered with an elastomeric material. The syntactic foam is a material made from hollow spherical particles, for example, glass spheres, dispersed in a binder, for example, epoxy. Rubber is used as an elastometric covering material covering the syntactic foam core.
In U.S. Pat. No. 4,407,368, Erbostoesser described an improved ball sealer having a solid core covered by a polyurethane coating. Another two-layer ball sealer was introduced by Doner, et al. in U.S. Pat. No. 4,505,334 in which a thermostatic filament is wrapped around a core, after which the material is cured, and having an optional elastomeric outer covering.
Further two-layer ball sealers are described in U.S. Pat. No. 4,702,316 to Chung et al, in which a ball sealer is described that is constructed from a polymer compound covered with a thin elastomer coating. In U.S. Pat. No. 5,253,709, Kendrick et al. describe a ball sealer having a deformable shell defining a central core filled with non-deformable particulate matter that can flow with the deformable shell yet, as it consolidates under fluid flow pressure, cause the outer shell to bridge over the perforation opening.
A rigid hollow core ball sealer is described in U.S. Pat. No. 5,485,882, to Bailey et al., entitled Low-Density Ball Sealer for Use as a Diverting Agent in Hostile Environment Wells, issued, Jan. 23, 1996. Bailey's ball sealers are formed from two pieces of high-strength materials that snap together to form a hollow-core sphere. The preferred material for Bailey's ball sealers include high-strength aluminum and high-strength thermoplastic and may include a protective coating to protect the aluminum against certain solvents found in some treatment fluids.
A degradable ball sealer is described in U.S. Pat. No. 6,380,138, to Ischy et al., entitled Injection Molded Degradable Casing Perforation Ball Sealers Fluid Loss Additive and Method of Use. Ischy's ball sealers are formed from a mixture of a soluble filler material and adhesives, and have the characteristic of softening slightly in the presence of a stimulating fluid thereby ensuring a solid contact through a controlled surface deformation. Ischy's ball sealers remain intact at near surface temperatures, i.e., the temperature of injected treatment fluid, but degrade when subjected to higher temperatures such as those expected after a return of natural well bore temperatures at the conclusion of a treatment.
From the foregoing it will be apparent that while ball sealers have been successfully designed to provide various desirable capabilities, there remains a need for improvement in ball sealers that can produce efficient seals with a variety of perforation shapes.